1. Field of the Invention
The present invention relates to a dielectric ceramic material which is advantageously employed in a monolithic ceramic electronic element, such as a monolithic ceramic capacitor comprising an internal electrode formed of a base metal such as nickel or copper, and to a monolithic ceramic electronic element which is formed of the dielectric ceramic material.
2. Background Art
Miniaturization and cost reduction of monolithic ceramic electronic elements are now in progress. For example, a ceramic layer has been thinned in such a ceramic electronic element, and a base metal has been employed as an internal electrode. In a monolithic ceramic capacitor, which is a typical example of a monolithic ceramic electronic element, a dielectric ceramic layer as thin as approximately 3 xcexcm has been formed and a base metal such as nickel or copper has been employed as a material for producing an internal electrode.
However, it is known that when a dielectric ceramic layer becomes thin, the layer is affected by a strong external electric field, exhibiting a great variation in dielectric constant per unit of temperature change. Therefore, there has been demand for a dielectric ceramic material exhibiting high reliability in a strong electric field, which material constitutes a dielectric ceramic layer.
Such a dielectric ceramic layer may be formed of an ABO3 perovskite dielectric ceramic material. Usually, the ceramic material comprises crystal grains of core-shell structure. A crystal grain of core-shell structure comprises a core portion and an outer surface shell portion that differs in crystal structure and composition.
Such a core-shell structure is created during sintering of a ceramic material in such a manner that a shell portion is formed through diffusion of an additional componentxe2x80x94usually a rare earth elementxe2x80x94from the surface of a crystal grain which serves as a core. In a conventional thin dielectric ceramic layer, diffusion of a rare earth element in a shell portion enhances reliability of the shell portion. As a result, reliability of the entire ceramic layer is secured.
In more than rare instances, however, grains of core-shell structure produced have excessively thin shells because of uneven diffusion or dispersion of the rare earth element. When such a ceramic material comprising crystal grains of core-shell structure is employed for forming a dielectric ceramic layer of a monolithic ceramic electronic element and the ceramic layer is as thin as 3 xcexcm or less, the ceramic layer will have some portions of low reliability. As a result, reliability of the monolithic ceramic electronic element may be lowered.
In view of the foregoing, an object of the present invention is to provide a dielectric ceramic material which can solve the aforementioned problem, and a monolithic ceramic electronic element which is formed of the dielectric ceramic element, such as a monolithic ceramic capacitor.
A more specific object of the present invention is to provide a dielectric ceramic material having a non-core-shell structure, in which the change in dielectric constant with temperature is small; and a monolithic ceramic electronic element which comprises a thin dielectric ceramic layer having a thickness of about 3 xcexcm or less, which has small size, high capacitance and high reliability, and which is produced at low cost.
Accordingly, the present invention provides a dielectric ceramic material comprising a primary component represented by the formula ABO3 and a rare earth element serving as an additional component, A in the formula representing Ba and optionally Ca and/or Sr, B in the formula representing Ti and optionally Zr and/or Hf, and O in the formula representing oxygen.
When the mean density of a rare earth element in an arbitrary crystal grain i of the crystal grains that constitute the ceramic material is represented by Di, the mean density of the rare earth element in the entirety of the ceramic material is represented by D, the standard deviation of the density of the rare earth element in the crystal grain i is represented by Si, the number of crystal grains satisfying the relation 0.5xe2x89xa6Di/D is represented by M, the number of the crystal grains constituting the ceramic material is represented by N, and the number of the crystal grains satisfying the relations 0.5xe2x89xa6Di/D and Si/Dxe2x89xa60.3 is represented by L, the ceramic material satisfies the following relations:
0.7xe2x89xa6M/N (i.e., the density of the rare earth element is uniform among crystal grains) and
0.8xe2x89xa6L/N (i.e., the density of the rare earth element is uniform within a crystal grain).
It must be noted that the aforementioned crystal grain i is not a specific crystal grain but an arbitrary crystal grain in the dielectric ceramic material. That is, when the number of the crystal grains that are present in the material is N, i may be an integer of 1 to N inclusive. Thus, for each one of the crystal grains that constitute a dielectric ceramic (number of grains: N), there can be calculated the mean density of the rare earth element (Di) in the crystal grains and the standard deviation of the density of the rare earth element in the crystal grains.
In the dielectric ceramic of the present invention, preferably, the mean size of the crystal grains is about 0.05-0.7 xcexcm, and the standard deviation of the size of the crystal grains is about 30% or less the mean grain size.
The present invention also provides a monolithic ceramic electronic element which is formed of the aforementioned dielectric ceramic material. More particularly, the present invention provides a monolithic ceramic electronic element comprising a laminate comprising a plurality of laminated dielectric ceramic layers and internal electrodes which are formed along specific interfaces between two dielectric ceramic layers adjacent to each other, wherein the dielectric ceramic layer is formed of the dielectric material.
In the monolithic ceramic electronic element of the present invention, the internal electrode preferably comprises nickel or copper, or an alloy thereof.
The present invention can be advantageously applied to, in particular, a monolithic ceramic capacitor. In such a case, the monolithic ceramic electronic element of the present invention further comprises first and second external electrodes which are formed on the outer surfaces of a laminate, wherein a plurality of internal electrodes are formed such that the internal electrodes overlap one another with respect to the vertical direction (stacking direction) of the laminate, and the internal electrodes which are electrically connected to the first external electrode and the internal electrodes which are electrically connected to the second external electrode are alternately provided in the stacking direction.
In the dielectric ceramic material of the present invention, crystal grainsxe2x80x94in which a rare earth element which exhibits effects on enhancement of reliability is almost uniformly dispersedxe2x80x94are present over the entirety of the dielectric ceramic material, and thus local segregation does not occur. Therefore, the ceramic material exhibits high reliability, and reliability can be secured from product to product.
In the dielectric ceramic material, the mean size of the crystal grains is about 0.05-0.7 xcexcm, and the standard deviation of the size of the crystal grains is about 30% or less the mean grain size, and thus dependence of the dielectric constant of the material on electric field is suppressed. In addition, in the case in which the ceramic material is applied to a monolithic electronic element, even when the thickness of a dielectric ceramic layer is about 3 xcexcm or less, temperature dependence of the dielectric constant of the electronic element can be suppressed.